1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and more particularly relates to a transparent heat-sensitive recording material which has a highly transparent heat-sensitive layer comprising solid particles of relatively large particle diameter dispersed therein and which has a high color forming sensitivity.
2. Description of the Related Art
A heat-sensitive recording method has various advantages, such as (1) it requires no developing process, (2) the recording material is almost the same as plain paper if the substrate is paper, (3) handling is easy, (4) the color-developing density is high, (5) the recording apparatus is simple and inexpensive, and (6) no noise is generated when recording is performed. Accordingly, its use is now rapidly spreading in the field of facsimiles and printers, and the scope of application of heat-sensitive recording is also widening in the labeling field, such as POS. Recently, because of this background, in order to meet the requirements of multicoloration or for use in an overhead projector (hereinafter abbreviated as "OHP"), a transparent heat-sensitive recording material capable of directly recording by means of a thermal head was developed. An example of the recent development is a transparent heat-sensitive recording material comprising a substrate and a layer formed thereon by a process comprising the steps of coating the substrate with a coating liquid comprising a dispersion prepared by emulsifying microcapsules, which contain a colorless or a light-colored electron donating dye precursor, and a color developer dissolved in an organic solvent, which is sparingly soluble or insoluble in water, and drying the resulting coating (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-45,084 and 63-265,682). However, it has become evident that this type of transparent heat-sensitive recording material has the drawback that the unused recording material has poor storage stability and that the density of the color-developing portion changes markedly with the passage of time.
Generally, since the transparency of a transparent heat-sensitive recording material can be secured by the prevention of light scattering in the recording layer, measures hitherto employed have included, for example, reducing the particle diameter of insoluble particles such as the particles of a color developer and increasing the proportion of a binder in order to prevent the change in refractive index due to void formation in the recording layer.
For the prevention of light scattering, the particle diameter is generally required to be less than 0.2 .mu.m, because the opacity caused by light scattering can be visually observed if a particle diameter is 0.3 .mu.m or more. However, the use of fine particles having diameters of less than 0.2 .mu.m is associated with problems in handling, for example, difficulty in dispersing these particles uniformly in a coating liquid to form a recording layer, and also problems such as decreased transparency due to the formation of large crystals in the coating layer because the heat generated by dispersing the fine particles causes re-flocculation of the particles. An additional problem is that although the transparency is improved by increasing the proportion of binder, the increase in the proportion of binder leads to a remarkable decrease in the sensitivity.
As described above, hitherto, a heat-sensitive recording material having excellent properties, such as high sensitivity, required for a heat-sensitive recording material along with a high level of transparency has not been achieved.